The postinstall script causes the installation to fail because the last
step that copies files for Xcode is broken. Two details can cause the
command to fail:
  1. The XCODE_MISC_DIR value has a space. Without quotes in the cp
     command, cp will just complain that this is an invalid syntax.
  2. The source of the cp is a directory with two subdirectories.
     We actually want the files for either Xcode 3 or Xcode 4 to be copied.
     Using xcodebuild -version, we check for the Xcode version and
     select which of xcode/3/* or xcode/4/* should be the source
     of the copy.

Fixes #5874.

R=golang-dev, minux.ma, adg
CC=golang-dev
https://golang.org/cl/10893044